Caps for containers are usually produced in that blanks that are punched out from sheet metal plates are deep drawn in a deep drawing die, whereby approximately crucible-shaped moulded blanks are formed with a ceiling- and an edge portion. Of course, a compression of the material in the edge portion takes place in this due to the reduction of the diameter. Because the used material, steel or aluminium in particular, has a given texture, small projections are formed on the free edge in such a drawing process. So called scallops or ears are formed (earing).
In caps for containers that receive delicate material or that are filled with a corrosion-promoting content, it is known to paint the inner side of such caps or to provide it with another coating, for instance with film material. The coating takes place on the sheet metal already, i.e. on the sheet metal web or the sheet metal plates that are to be punched. In this, fine pieces of fluff or hair-like formations are formed on the cylindrical or flangeless edge of the deep drawn cap, which are considered to be extremely disadvantageous. The reason is on the one hand that such “fluffs” strongly soil the die. On the other hand, they can contaminate the contents of a container in the later utilisation of the cap.
A typical deep draw die for deep drawing flat blanks in order to form caps provides a drawing bell and a drawing core, round about which the drawing bell forms the crucible-shaped moulded blank. Because of the already described diameter reduction, creases can be formed on the edge. Therefore, such a deep draw die provides a so-called blank holder, which bears against the edge region under a spring force. In this, the inner side of the edge region is located on the blank holder, and thereby it bears against the layer of paint or film material. In the art, it is assumed that the breaking of the material at the end of the punching process, which is inevitable with harder sheet metal in particular, is the reason of the fraying of the layer.
From EP 0 595 417 B1, a spring device for a blank holder of a drawing die has become known, by which the force of the blank holder applied to the flange of the moulded blank is reduced in the progression of the deep drawing process. As is well known, the pressure intensity between blank holder and moulded blank increases in the progression of the deformation process even at constant force on the blank holder, because the area of the edge region of the moulded blank that co-operates with the blank holder decreases progressively. By successive reduction of the force on the blank holder, it is intended to keep the pressure intensity approximately constant in the known case.
From U.S. Pat. No. 5,433,099, a method has become known for deep drawing blanks which are punched out of sheet metal that is painted or coated with film material, for example made of steel or aluminium, into flangeless moulded blanks, wherein the blanks are deformed to a crucible-shaped part with a flangeless cylindrical edge, round about a drawing core by means of a drawing bell of a drawing die, and a predetermined spring force is applied to the side of the edge opposite to the drawing bell during the forming of the edge of the blanks by means of a blank holder. The spring force applied to the blank holder is spontaneously reduced substantially to zero shortly before the end of the drawing process.
From EP-A-0 595 417, a drawing die for deep drawing blanks has become known, with a drawing bell, a drawing core, a blank holder or a pneumatic spring which applies a spring force to the blank holder.